This invention is concerned with the chemical reaction of compounds having silicon bonded hydroxy or alkoxy groups by way of condensation reactions.
It is well known that chain extension and crosslinking reactions of silicon containing compounds may be achieved readily by way of condensation of silicon bonded hydroxy, alkoxy or other condensable groups present in the compound or formed therein, for example, during the course of condensation. These reactions may be, for example, according to the schemes:
2xcx9cSiR2OHxe2x86x92xcx9cSiR2OSiR2Oxcx9c+H2O, or
xcx9cSiR2OH+ROSiR2ORxe2x86x92xcx9cSiR2OSiR2OR+ROH where R represents a substituted or unsubstituted, saturated or unsaturated hydrocarbon group.
Such reactions are employed in commerce especially in the manufacture of polydiorganosiloxanes of elevated molecular weights and in the formulation of a variety of silicone compounds employed in one part or multi part form for a wide range of uses in which the compound is required to cure in situ to a crosslinked condition. The polymerisation of silanol containing oligomers HO (SiMe2O)nH (where Me represents a methyl group CH3 and n has a value of, for example, from about 4 to about 40) to form silicone polymers of elevated molecular weight (i.e. n has a value in excess of 1,000) by condensation is an extremely important part of the process for manufacture of silicone polymer materials having viscosities varying from those of fluids to those of gums. It is well known that this chain extension process may be carried out batchwise or continuously. Typically the reaction is conducted in the presence of one or more catalyst materials.
Various acidic and basic materials are known for use as catalysts for reaction of organosilicon materials via silanol condensation reaction, for example potassium hydroxide, ammonium hydroxide, barium hydroxide, acid clays, sulphonic acids, and phosphazene bases. However, these catalysts tend to catalyse other reactions simultaneously in addition to condensation reactions and one consequence can be the presence of significant proportions of cyclic siloxanes in the product. Also, catalysts are required to perform consistently and should be capable of removal (along with other undesirable residues) from the product, and those for use in continuous production processes are required to perform rapidly. One type of material proposed for use in the manufacture of silicones of elevated molecular weight is a phosponitrile chloride. Although this material has a number of advantages as a catalyst for the polymerisation of organosilicon materials, it is produced and used in chlorinated solvents which are regarded as potentially environmentally hazardous and thus require special handling. It is also rather difficult to neutralise consistently in the polymer production. Furthermore the phosphonitrile chloride is susceptible to hydrolysis and on prolonged exposure to water loses catalytic activity.
Thus, despite the many proposals for catalysis materials for such condensation reactions there remains a desire to provide a material which can serve as an effective catalyst, which can be prepared by a simple process and which does not leave, within the bulk of the reaction product, residues which are difficult to neutralise or remove.
Surprisingly we have now found that condensation of compounds having silicon bonded hydroxy or alkoxy groups may be achieved in presence of a catalytic amount of one or more materials providing a source of anions comprising at least one quadri-substituted boron atom and protons capable of interaction with at least one silanol group.
The present invention provides in one of its aspects a process for the condensation of a compound having a silicon bonded hydroxy or alkoxy group in the presence of a catalytic amount of one or more materials providing in the reaction mixture an anion comprising at least one quadri-substituted boron atom and protons capable of interaction with at least one of said silicon bonded hydroxy or alkoxy groups.
In a process according to the invention, it is important that the boron containing anion does not itself form a covalent bond directly to a silicon atom and that it does not decompose or rearrange to produce an anion which forms a covalent bond directly to a silicon atom. Suitable materials include those incorporating one or more boron atoms disposed within a grouping and several, for example ten or more, halogen atoms connected with each boron atom. The halogen atoms in such compound may be connected to boron atoms by linkages incorporating at least one carbon atom. The halogen atoms are preferably selected from fluorine, chlorine and bromine, the most preferred being fluorine. Preferred anions incorporate one or more atoms of boron having four organic substitutes thereon the most preferred being quadri-substituted borates. The organic substituents are suitably hydrocarbon groups. Three and preferably four of these hydrocarbon groups are preferably aromatic groups, and are preferably highly halogenated. Preferred halogenated hydrocarbons are pentafluorinated phenyl groups and his (trifluoromethyl) phenyl groups and preferred materials have four such groups bonded to each boron atom. One operative material is the tetrakis (pentafluoro phenyl) borate anion (otherwise herein referred to as the perfluorinated aryl borate ion) and the material is preferably employed as the acid of this anion namely H+ {(C6F5)4B}xe2x88x92. Other operative materials include anions having two quadri-substituted boron atoms for example, di-perfluoroinated aryl borate ions eg H+ {B(C6F5)3 CNB(C6F5)3}xe2x88x92. The preferred materials can be readily prepared from commercially available compounds by simple ion exchange techniques in innocuous solvents, for example, water or alcohol. We prefer to prepare the acids prior to introducing catalytic amounts of them to the reaction mixture.
Other suitable boron-containing anions for use in the process of the present invention include carboranes, for example of the formula: {CB9H10}xe2x88x92, {CB9X5H5}xe2x88x92, {CB11H12}xe2x88x92, and {CB11X6H6}xe2x88x92, wherein X represents fluorine, chlorine, bromine or iodine. Carboranes may contain boron atoms which are more highly substituted than quadri-substituted, eg penta-substituted and hexa-substituted, and for the sake of clarity, xe2x80x9cquadri-substitutedxe2x80x9d where used herein is intended to include those anions containing quadri-substituted and higher substituted boron atoms.
In a process according to the invention, one may employ any suitable compound having silicon bonded hydroxy or alkoxy groups. Preferred materials are silanes and siloxane compounds having at least one unit according to the general formula:
xe2x80x83RoaR1bR2cSiO(4xe2x88x92(a+b+c)/2)xe2x80x83xe2x80x83(i)
in which each Ro represents a hydroxy, alkoxy, alkoxyalkoxy or hydrocarbonoxy group having up to 10 carbon atoms, each R1 represents a hydrogen atom or a monovalent substituted or unsubstituted hydrocarbon group, each R2 represents a divalent substituted or unsubstituted alkylene, or oxyalkylene group which is linked for example to another unit of formula (i) or an atom of a polymeric material, as referred to below a has the value of 1, 2, 3, or 4, b has a value of 0, 1, 2 or 3, c has a value of 0, 1, 2 or three and a+b+c has the value of 1, 2, 3 or 4. Suitable groups Ro include, for example, hydroxy, methoxy, ethoxy, butoxy, phenoxy, and methoxyethoxy. Suitable groups R1 include, for example, hydrogen, alkyl groups for example methyl, ethyl, propyl, isobutyl, hexyl, dodecyl or octadecyl, alkenyl for example, vinyl, allyl, butenyl, hexenyl or decenyl, alkynyl for example propargyl, aryl for example phenyl, aralkyl for example tolyl or xylyl, substituted hydrocarbon groups for example trifluoropropyl, chloropropyl or chlorophenyl. Suitable groups R2 include for example, xe2x80x94(CH2)nxe2x80x94 where n has a value of 1, 2, 3 or more and xe2x80x94(OCH2CHR3)mxe2x80x94 where R3 represents H or xe2x80x94CH3 and m has a value of greater than about 5. The compounds having at least one unit according to the general formula (i) may be monomeric, oligomeric or polymeric. The monomeric materials are preferably silanes in which c has a value of 0 and a+b has the of value 4. The polymeric materials may be predominantly organic materials or predominantly siloxane materials. Examples of suitable predominantly organic materials are those in which one or more units of formula (i) is incorporated in an organic polymer via its divalent group R2. Examples of predominantly siloxane materials are polymers which incorporate units according to the general formula (ii) R1sSiO(4xe2x88x92s)/2 where R1 is as aforesaid and s has the value 0, 1, 2 or 3. Preferably, large proportions (preferably more than 80%) of these units are those where s has the value 2. If desired, these polymers may have one or more of the units of formula (i) attached via their divalent linkage R2 to a silicon atom of the polymer.
In a process according to the invention, the compound having silicone bonded hydroxy or alkoxy groups may condense with the same, another or several other compounds having silicon bonded hydroxy or alkoxy groups. By appropriate variation of the values a+b+c and of the groups Ro, R1 R2 one may cause condensation reaction to provide products of a variety of molecular sizes, functionalities and reactivities which are thus suitable for a wide range of uses. As mentioned, the compounds according to the general formula: RoaR1bR2cSiO(4xe2x88x92(a+b+c)/2) may have one or more groups Ro. In a process according to the invention, a first one of these compounds may be caused to combine with a second one of these compounds by way of condensation of one Ro group of each of the compounds. In this way, the first of these compounds may be employed to consume Ro groups of the second of the compounds and to introduce a desired grouping to the second compound. For example, in the case where the first compound is a silane of formula (i) and the second compound is a polymer having units of the formula (i), by appropriate selection of the values of a one may bring about chain extending, chain branching or chain terminating condensation reactions, in which pairs of groups Ro are consumed. Also by appropriate selection of values of a and values of b, groups R1 may be introduced into the chain or at its ends. Introduction of alkenyl groups in this manner is of interest as providing a route to reaction via their unsaturation.
Particularly suitable materials having silicon bonded hydroxy or alkoxy groups include for example the di-hydroxy or alkoxy xcex1,xcfx89-dihydroxy-polydiorganosiloxanes according to the formula HO (SiMe2O)nH where n has a value from about 4 to about 40 and diethoxymethylvinylsilane and the mono-alkoxy material according to the general formula: Me3SiO(SiMe2O)nSiMe3 (where Me represents the methyl group CH3 and n has a value from 0 to 100), ethoxydimethylvinylsilane, and methoxydimethylhexenylsilane and mixtures of two or more thereof.
In a process according to the present invention the compound or compounds having silicone bonded hydroxy or alkoxy groups are provided as a mass of material. In the case where manufacture of polymer is to be carried out, the mass is confined in a reaction vessel of the batch or continuous type. In the event one wishes to provide the groups Ro on the compound by conversion of other groups eg Cl or CN, this may be done as a separate step or less preferably in the reaction mass of material. If more than one compound having silicon bonded hydroxyl or alkoxy groups is to be employed, the compounds may be introduced to the reaction vessel in any desired order. Catalyst is introduced to the reaction mass in any desired order and condensation reaction conducted at any desired temperature and pressure. The reaction may be carried out at room or elevated temperature with or without reduced pressure. The catalyst may be used at a concentration of from 1 to 500 ppm by weight based on the total reactants. The amount used may be varied according to the temperature used for the reaction. At room temperature we prefer to employ from 100 to 500 ppm whereas for reactions at 80xc2x0 C. we prefer to employ 1 to 30 ppm.
If desired, various materials may be present in the reaction mixture, for example, solvents, reinforcing or extending fillers, co-catalysts, pigments, plasticisers, extenders or mixtures of any two or more thereof always provided they do not adversely influence the reaction.
The present invention is concerned with provision of catalysts for the homo- or co-condensation of materials having silicon bonded hydroxy, or alkoxy groups and especially but not exclusively with those which are particularly efficacious for the manufacture of higher molecular weight linear or branched polymeric organosilicon materials having desired pendant or terminal groups from silanols by batch or continuous processes. The catalyst materials employed in the present invention appear to catalyse the condensation reactions if and so long as condensable co-reactants are present. When such co-reactants are not present in sufficient quantities, these catalyst materials are capable of catalysing re-equilibration of the formed polymer to yield lower molecular weight polymer mixed with cyclic siloxanes. The catalytic activity may be terminated when it is no longer required, for example by neutralising the materials using a basic substance, such as an organic amine, or by heating to decompose the catalyst. This may be done at any stage of the process, for example when a desired viscosity has been achieved and before significant re-equilibration can take place.
A process according to the invention offers various advantages over prior known processes. The catalyst materials are stable to water and alcohol and their catalytic activity is not significantly reduced by exposure thereto. Preparation of the catalyst and the introduction of the catalyst to the polymerisation reaction without use of chlorinated solvents renders production and use of the catalyst more environmentally acceptable. The presence in the reaction product of undesirable residual catalyst and compounds derived therefrom is reduced not only due to absence of chloride ions but also due to the ease of neutralising the catalyst. Thus, for example, at least substantially linear polydiorganosiloxanes can be produced in a cleaner form due to the absence of chlorinated solvents from the catalyst and the process may be controlled so as to enable production of at least substantially linear polydiorganosiloxanes incorporating small proportions of cyclic silicones.
The word xe2x80x9ccomprisingxe2x80x9d where used herein is intended to embrace the notion of including as well as the notion of consisting of.
In order that the invention may become more clear there now follows a description of examples selected to illustrate the invention by way of example. In these examples unless the context states otherwise, the symbol Et represents the ethyl group, He represents the hexenyl group, Me represents the methyl group, Ph represents the phenyl group, Vi represents the vinyl group, all parts are by weight and all viscosities are determined at 25xc2x0 C. and expressed in centipoise (1 poise=0.1 Paxc2x7S).